I don't think the main problem is that they're distracting, in the drawing attention away from what you're writing sense. I think the main problem is that, for an unfamiliar editor, the autocomplete takes an unknown subset of keystrokes, meaning that common things like inserting a newline or navigating vertically can interrupt your train of thought.
Once you get familiar with a particular editor then these problems become less acute. What remains is the obscuring of other lines of code which can be fixed by making the autocomplete box only appear when you press tab.
Once you get familiar with a particular editor then these problems become less acute. What remains is the obscuring of other lines of code which can be fixed by making the autocomplete box only appear when you press tab.